Narrative:

I'm submitting this report as a reminder of the hazards associated with cumulonimbus buildups in the terminal area; even though the flight was minimally impacted. We were flying the ZZZZZ5 RNAV arrival into ZZZ. I can't remember exactly where on the arrival we were; possible flying direct to [an airport] or we may have been on an assigned heading at this point - it isn't really important. We were cleared to descent to FL240. As is usually the case in [area] in summer time; there were numerous buildups in the area; including a pretty large thunderstorm to the northeast of the airport. Fortunately this arrival comes in from the west; so we were VMC for the entire route. There was a small circuit breaker buildup directly in our path as we were leveling off at FL240. I didn't think much of it because I could tell that we would fly through near the top of it and be out the other side in just a few seconds. I expected it to be slightly bumpy; nothing more. However; we entered the cloud right as we were leveling off; and I encountered the worst turbulence of my career. The aircraft was immediately knocked into an approximate 20-30 degree left wing down attitude and we went below our assigned altitude by about 200 ft. It caught us completely off guard. We had a deadhead and a jumpseat crew member sitting in the seats. Fortunately they were strapped in. As expected we were out of the cloud almost as soon as the incident started - probably no more than a couple of seconds. I disconnected the autopilot to manually get us back to our assigned altitude of FL240. We reautomated and rest of the arrival and approach were uneventful. I am not certain that this technically qualifies as severe turbulence or not but it was certainly borderline. In hind sight it might have been a good idea to request a deviation around the cloud; but as I said it wasn't very big and I really didn't expect it to be as bad as it was. Good lesson learned.

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Original NASA ASRS Text

Title: B767-300 Captain reported severe turbulence; causing a loss of aircraft control.

Narrative: I'm submitting this report as a reminder of the hazards associated with cumulonimbus buildups in the terminal area; even though the flight was minimally impacted. We were flying the ZZZZZ5 RNAV arrival into ZZZ. I can't remember exactly where on the arrival we were; possible flying direct to [an airport] or we may have been on an assigned heading at this point - it isn't really important. We were cleared to descent to FL240. As is usually the case in [area] in summer time; there were numerous buildups in the area; including a pretty large thunderstorm to the northeast of the airport. Fortunately this arrival comes in from the west; so we were VMC for the entire route. There was a small CB buildup directly in our path as we were leveling off at FL240. I didn't think much of it because I could tell that we would fly through near the top of it and be out the other side in just a few seconds. I expected it to be slightly bumpy; nothing more. However; we entered the cloud right as we were leveling off; and I encountered the worst turbulence of my career. The aircraft was immediately knocked into an approximate 20-30 degree left wing down attitude and we went below our assigned altitude by about 200 ft. It caught us completely off guard. We had a deadhead and a jumpseat crew member sitting in the seats. Fortunately they were strapped in. As expected we were out of the cloud almost as soon as the incident started - probably no more than a couple of seconds. I disconnected the autopilot to manually get us back to our assigned altitude of FL240. We reautomated and rest of the arrival and approach were uneventful. I am not certain that this technically qualifies as severe turbulence or not but it was certainly borderline. In hind sight it might have been a good idea to request a deviation around the cloud; but as I said it wasn't very big and I really didn't expect it to be as bad as it was. Good lesson learned.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.